1. Field of the Invention
The present invention pertains to an improved process for producing a carbonaceous pitch product having a mesophase content ranging from about 50 to 100%, which is suitable for carbon fiber manufacture. More particularly, the invention relates to a process for making mesophase-containing pitch capable of producing carbon fibers having enhanced properties, by use of an oxidatively reactive sparge gas during heat treatment of mesophase precursor.
2. The Prior Art
In recent years extensive patent literature has evolved concerning the conversion of carbonaceous pitch feed material into a mesophase-containing pitch which is suitable for the manufacture of carbon fibers having desirable modulus of elasticity, tensile strength, and elongation characteristics.
U.S. Pat. No. 4,209,500 which issued to Chwastiak on June 24, 1980 is directed to the production of a high mesophase pitch that can be employed in the manufacture of carbon fibers. This patent is one of a series of patents pertaining to a process for producing mesophase pitches suitable for carbon fiber production. Each of these patents broadly involves heat treating or heat soaking the carbonaceous feed while agitating and/or passing an inert gas therethrough so as to produce a more suitable pitch product for the manufacture of carbon fibers.
As set forth in the Chwastiak patent, earlier U.S. Pat. Nos. 3,976,729 and 4,017,327 issued to Lewis et al involve agitating the carbonaceous starting material during the heat treatment. The use of an inert sparge gas during heat treatment is found in U.S. Pat. Nos. 3,974,264 and 4,026,788 issued to McHenry. Stirring or agitating the starting material while sparging with an inert gas is also disclosed in the McHenry patents.
The Chwastiak patent additionally discloses the prior state of the art, as well as the Lewis et al and McHenry patents discussed above, and these disclosures by Chwastiak in Column 1, line 13 to Column 2, line 50, are incorporated herein by reference. Both the Lewis et al and McHenry proposals to promote mesophase formation had serious limitations, according to Chwastiak, in that their pitch products tended to segregate into two phase systems which hampered subsequent spinning operations.
Chwastiak proposed a mesophase-producing process wherein a single phase product resulted. The process requires both agitation and an inert gas sparge, the improvement resulting, according to Chwastiak, by passing the inert gas through the pitch at a rate of at least 4 standard cubic feet per hour (SCFH) per pound of pitch. In the illustration the heat treatment required 44 hours. It would be advantageous therefore to provide a process which did not require such a high rate of inert gas flow and which also could be accomplished in less time without deleteriously affecting the pitch product.
In published German Patent Application No. 3305-055-A (Nippon Oil KK) there is disclosed a process wherein a pitch feed is initially heat treated at 370.degree. to 420.degree. C. in a stream of inert gas for 5 to 20 hours under atmospheric or reduced pressure. Subsequently, an oxidant gas such as air or oxygen is passed through the pitch at 200.degree.-350.degree. C., one atmosphere pressure, at a flow rate of 1.0 to 3.5 SCFH for 10 minutes to 2 hours. No increase in mesophase content was reported as a result of the separate treatment with the oxidant gas.
Koppers Co. Inc. has published DT No. 2221707-Q and DT No. 2357477 patent applications, which disclose manufacture of isotropic carbon fibers wherein the starting material is first reacted with oxygen and then vacuum distilled, to remove non-oxidized lower-boiling components.